In recent years, in fields to which alternating-current motors are applied such as the fields of industrial machines, home electric appliances, automobiles, and the like, the number of examples in which a permanent magnet synchronous motor is driven and controlled by an inverter has increased so as to replace conventional methods in which an induction motor is driven and controlled by an inverter.
Permanent magnet synchronous motors are known as having higher efficiency than induction motors for, for example, the following reasons: permanent magnet synchronous motors do not need excitation currents because a magnetic flux is established by a permanent magnet; no secondary copper loss occurs in permanent magnet synchronous motors because no electric current flows into the rotor; permanent magnet synchronous motors are able to effectively obtain a torque by, in addition to a torque generated by a magnetic flux established by a permanent magnet, making use of a reluctance torque that utilizes a difference between magnetic resistances in the rotor. In recent years, applying permanent magnet synchronous motors to electric power converting apparatuses used for driving railway electric cars have also been considered.
Patent Document 1: Japanese Patent Application Laid-open No. H7-227085